Audiometers for testing the hearing of human subjects have been in widespread use for many years. Typically, an audiometer includes a test signal generator component for generating audio signals, and one or more transducers for converting the signals to audible sound in the form of test tones or speech. The test signal generator is often controlled by a computer or microprocessor so as to enable audio testing of subjects to take place automatically without a highly trained operator in attendance.
Test sounds are typically presented to the test subjects at precise absolute sound pressure levels for accuracy, standardization, and comparison purposes. Due to variability in component values and characteristics, however, test signal generators may output significantly different output levels and different transducers may produce significantly different absolute sound pressure levels when supplied with an electrical signal of a uniform value. For these reasons, the test signal generator and transducers of prior art audiometers are calibrated together. That is, the test signal generator is matched to a single transducer set and adjusted to give the desired absolute sound pressure levels. An example of such a prior art audiometer apparatus is disclosed in U.S. Pat. No. 6,468,224 to Foreman, hereby fully incorporated herein by reference.
A drawback of these prior art audiometer systems arises when transducers are lost or damaged. The test signal generator and the specific transducers that are to be used with the test signal generator in these prior systems must have been at least once physically connected with each other and adjusted to ensure the accuracy of the sound pressure levels output by the transducers. Consequently, if the transducer is lost or damaged, a new transducer must be matched and re-calibrated with the test signal generator. This typically either involves shipping the audiometer to the original manufacturer or authorized repair location to enable the recalibration and then returning the newly calibrated audiometer to the service location or requires an on-site visit by a skilled repair technician with the proper equipment. Attempts have been made to address this problem by including a limited number of spare transducers, and providing the audiometer with multiple tables of correction values stored therein that can be switched automatically or manually when a spare transducer is substituted. This, however, is not a completely satisfactory solution since the transducers and test signal generator must still have been once physically connected. Also, once supply of spare transducers is depleted, new transducers need to be matched by recalibration on site or at a repair facility. For the user, if a backup audiometer with calibrated transducer is not available, the result is extended downtime when the audiometer is not available for use. Moreover, it is often not economically feasible to maintain backups because of the relatively high cost of precision automatic audiometers.
What is still needed in the industry is an audiometer system that enables any transducer to be used with the test signal generator without having been specifically matched.